Understanding cancer

AIDS and Kaposi's Sarcoma

People who develop AIDS after being infected with the human immunodeficiency virus (HIV) are at high risk for developing a specific type of cancer called Kaposi's sarcoma. Kaposi's sarcoma is a malignant tumor of blood vessels located in the skin. This type of cancer is not directly caused by HIV infection. Instead, HIV causes an immune deficiency that makes people more susceptible to viral infection. Infection by a virus called KSHV (Kaposi's sarcoma-associated herpesvirus) then appears to stimulate the development of Kaposi's sarcoma.

Bacteria and Stomach Cancer

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Viruses are not the only infectious agents that have been implicated in human cancer. The bacterium H. pylori, which can cause stomach ulcers, has been associated with the development of stomach cancer. People infected with H. pylori are at increased risk of developing stomach cancer. Research is under way to define the genetic interactions between infectious agents and their hosts that may explain why cancer develops.

Heredity and Cancer

Cancer is not considered an inherited illness because most cases of cancer, about ninety to ninety-five percent, occur in people with no family history of the disease. However, a person's chances of developing cancer can be influenced by the inheritance of certain kinds of genetic alterations. These alterations tend to increase an individual's susceptibility to developing cancer in the future. For example, about five to ten percent of breast cancers are thought to be due to inheritance of particular form(s) of a "breast cancer susceptibility gene."

Heredity Can Affect Many Types of Cancer

Inherited mutations can influence a person's risk of developing many types of cancer in addition to breast cancer. For example, specific inherited mutations have been described that increase a person's risk of developing colon cancer, kidney cancer, bone cancer, skin cancer, or other specific forms of cancer. But these hereditary conditions that increase a person's risk of developing specific types of cancer are thought to be involved in ten percent or fewer of all cancer cases.

Genetic Testing

Laboratory tests can determine whether a person carries some of the genetic alterations that can increase a person's risk of developing certain cancers. For example, women who inherit certain forms of a gene called BRCA1 have an elevated risk of developing breast cancer. For women with a family history of breast cancer, taking such a test may relieve uncertainty about their future risk. However, the information obtained from genetic tests is often complex and difficult to interpret. The decision to undergo genetic testing should therefore be a personal, voluntary one and should only be made in conjunction with appropriate genetic counseling.

Cancer Risk and Aging

Because cancer usually requires a number of mutations, the chances of developing cancer increase as a person gets older because more time has been available for mutations to accumulate. Because people are living longer today than they did 50 or 100 years ago, they have a longer exposure time to the factors that may start gene changes that lead to cancer.

Genes and Cancer

Chemicals (e.g., from smoking), radiation, viruses, and heredity all contribute to the development of cancer by triggering changes in a cell's genes. Chemicals and radiation act by damaging genes, viruses introduce their own genes into cells, and heredity passes on alterations in genes that make a person more susceptible to cancer. Genes are inherited instructions that are regions within DNA molecules. Each gene allows a cell to make a specific product - in most cases, a particular kind of protein. Genes are altered, or "mutated," in various ways as part of the mechanism by which cancer arises.

DNA Structure

Genes reside within large DNA molecules, which are composed of two chemical strands twisted around each other to form a "double helix." Each strand is constructed from millions of chemical building blocks called "bases." DNA contains only four different bases (abbreviated A, T, G, and C), but they can be arranged in any sequence. The sequential order of the bases in any given gene determines the message the gene contains, just as the letters of the alphabet can be combined in different ways to form distinct words and sentences.

DNA Mutation

Genes can be mutated in several different ways. The simplest type of mutation involves a change in a single base along the base sequence of a particular gene - much like a typographical error in a word that has been misspelled. In other cases, one or more bases may be added or deleted. And sometimes, large segments of a DNA molecule are accidentally repeated, deleted,or moved.

Gene Mutations and Cancer

Mutations in genes that control normal cell proliferation can lead to cancer. These mutations can be created by DNA-damaging carcinogens such as cigarette by-products and radiation. However, some cancer-causing mutations are simply spontaneous errors that appear in normal DNA molecules when cells duplicate their DNA prior to cell division. The mutations that contribute to the development of cancer affect three general classes of gene: oncogenes, tumor suppressor genes, and DNA repair genes.

Oncogenes

The first group of genes implicated in the development of cancer are damaged genes, called "oncogenes." Oncogenes are genes whose presence in certain forms and/or overactivity can stimulate the development of cancer. When oncogenes arise in normal cells, they can cause the cells to become malignant. Oncogenes contribute to the development of cancer by instructing cells to make proteins that stimulate excessive cell growth and division.

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